Remediation Effects And Mechanisms Of Green Waste Compost On Pb And Cd Contaminated Soil

Green waste compost(GWC)is sourced from weeds,flowers,leaves and other green wastes through aerobic composting process of degradation and decomposition.GWC has the characteristics of rich organic matter with high nutrient content,and is barely involved in the release of exogenous pollutants,showing broad potential in the soil remediation of heavy metals.This study selected peat as a comparative material,the adsorption mechanism of Pb2+ and Cd2+ were revealed by material property characterization and adsorption experiments.Soil simulation test was carried out to figure the remediation effect of Pb and Cd under different conditions by GWC.Pot experiment was implemented to study the improvement of GWC on soil physicochemical properties and the growth of pak choi.Finally,field experiment was carried out on agricultural soil that polluted by various heavy metals.The application prospect of GWC in actual production was assessed.This study aimed at offering a new way of applying GWC,providing scientific guidance and theoretical basis for passivating heavy metals,improving soil quality,and promoting crop growth using GWC as a soil conditioner.The main findings are as follows:(1)Pb2+ was observed to be adsorbed on the surface of the GWC by scanning electron microscope image and energy dispersive analysis(SEM-EDX).The results of Fourier Transform infrared spectroscopy(FTIR)analysis showed that the main functional groups involved in the complexation on the surface of organic matter were carboxyl group,hydroxyl group and amide group.Diffraction of X-Rays(XRD)analysis detected that Pb2+ formed a new precipitate of Pb As2O6 in GWC-added aqueous solution.The adsorption kinetics of GWC and peat conformed to the pseudo-second-order kinetic equation,indicating that the adsorption process was mainly controlled by chemical reactions.Both the Freundlich and Langmuir isotherm equations fit the experimental results well,indicating that the adsorption process of Pb2+ and Cd2+ in aqueous solution has both monolayer and heterogeneous surface conditions.In conclusion,the adsorption mechanisms of GWC onto Pb2+ and Cd2+ in aqueous phase were mainly: electrostatic adsorption,chemical precipitation and complexation through hydroxyl,carboxyl and amide groups.(2)Three different soil initial conditions were set: bulk density(1.0、1.2、1.4 g/cm3),water content(10%,15%,20%),the alternative freezing and thawing.The passivation effect of GWC and peat on Pb and Cd was enhanced with the decrease of soil bulk density.In addition,in the soil with bulk density of 1.0 g/cm3,the passivation effect of GWC is better than that of peat.The changes of Pb and Cd speciation in GWC treatment were more sensitive to the changes of water content,and high-water content could promote the decrease of acid soluble Pb form.The peat treatment did not show the significant changes of Pb and Cd speciation under different water contents.Freeze-thaw alternating could promote the decline of acid soluble Pb and Cd contents into GWC and peat treatment.Under freeze-thaw alternating,the content of acid-soluble Pb and Cd in the GWC treated soil decreased by 5.5%and 7.1%,compared with room temperature,and the residual contents significantly increased by 10.3% and 14.1%,respectively.The acid soluble Pb and Cd content in peat treatment decreased by 24.4% and 22.0%,and the residual content increased by 11.5% and22.5%,respectively.The results showed that the effect of alternative freezing and thawing onto passivation of Pb and Cd by peat was greater than that of GWC.(3)In the pot experiment,the application of GWC and peat significantly reduced the content of Pb in soil pore water,while the content of Cd increased.The residual content of Pb and Cd increased with the application of GWC,which proved that both GWC and peat had significant passivation effects on Pb and Cd in soil,and the effect of mixed application was better than that of single.Soil p H,organic matter and chemisorption were the main factors controlling the transformation of Pb and Cd speciation.Peat was more significant for promoting dehydrogenase and catalase activities;GWC was more significant for enhancing urease and phosphatase activities.Peat and GWC significantly reduced Cd bioavailability in soil.However,the single application of GWC significantly increased the Pb content in the shoots of Brassica chinensis L.,indicating that GWC could promote the absorption and translocation of Pb from the roots to the shoots of pak choi.(4)In the field experiments after the application of GWC,the Triticum aestivum L.yield increased from 263.9 kg/667 m2 to 325.3 kg/667 m2;the Brassica campestris ssp.Chinensis L.increased from 1001 kg/667 m2 to 2623 kg/667 m2.Pb contents in the roots,stems and grains of wheat decreased from 9.66 mg/kg,2.18 mg/kg,0.8 mg/kg to 4.32 mg/kg,0.99mg/kg,0.61 mg/kg,respectively;Cd contents decreased from 2.43 mg/kg,0.14 mg/kg and0.29 mg/kg to 0.37 mg/kg,0.09 mg/kg,0.20 mg/kg.Pb content in shoots and roots of pak choi decreased from 12.34 mg/kg,5.26 mg/kg to 6.22 mg/kg,1.99 mg/kg;Cd content decreased from 0.35 mg/kg,0.77 mg/kg to 0.17 mg/kg,0.24 mg/kg,respectively.The results above proved that GWC,as an organic fertilizer,can inhibit the accumulation of Pb and Cd in crops while promoting the growth,and play an important role in reducing the risk of heavy metals transmission in the food chain.(5)The application of GWC has a significant passivation effect on the available Pb and Cd in the field surface soil(0-20 cm).The passivation effects follow: Pb> Cd.At the same time,GWC enhanced the migration of heavy metals in the topsoil.Among them,the influence depth on the migration of Pb is limited to the soil layer of 20-30 cm;while the migration depth of Cd can reach the soil layer of 60 cm.Therefore,in soils with serious Cd,if the groundwater depth is shallow,the use of GWC may cause potential ecological risks,which should be carefully considered.(6)After applying GWC,the soil bacterial community structure was changed.The Shannon index,observed species index and Chao1 index of soil were significantly lower than those of soil without GWC.At the same time,it also caused a large difference in the abundance of OUTs.GWC has a total of 11872 OTUs with 8450 unique OTUs,accounting for 71.2% of the total species.A total of 12273 OTUs in the original soil with 8851 unique OTUs,and 72.1% of the total species.But instead of changing the type of dominant phylum,the abundance of the dominant phylum was changed.The application of GWC increased the diversity and abundance of soil fungal community,the observed species index and Chao1 index of the soil were significantly higher than those of the soil without GWC(P<0.05),and the number of all taxonomic levels except the kingdom was improved.The species of fungal dominant phyla did not change,but also only changed their abundance.Soil p H,organic matter content,total N and acid soluble Pb and Cd content were the environmental factors that had a significant impact on soil bacterial and fungal communities.Application of GWC significantly increased the abundance of Nocarioides at the genus level of the bacterial community,Mycosphaerella and Macrophoma at the genus level of the fungal community,thereby enhancing the influence of Pb,Adsorption passivation of Cd.In conclusion,GWC could be considered as a sustainable and environmentally friendly organic fertilizer,improving soil fertility and promote crop growth.At the same time,it could be used as an amendment for the soil remediation of heavy metal pollution.